The present invention relates to a process for positioning a point belonging to the cutting zone of a tool in a digitally controlled lathe. It also relates to an apparatus making it possible to perform this process. It applies to the very precise positioning of a point of a cutting zone of a tool in a digitally controlled, very high precision lathe.
It is known that in a digitally controlled lathe, the profile of the parts to be machined is obtained from a planar trajectory of the tool defined in a fixed working mark or station, linked with the lathe frame. This mark is defined by the rotation axis of the part (chuck axis) and by a plane perpendicular to said rotation axis. If several tools are necessary for machining a part, the programmed trajectory corresponds to that of the fixing point common to each tool on the tool-holder capstan or turret.
A first method for positioning a point of a tool belonging to the cutting zone consists of previously measuring the length of each tool and then storing said length in the digital control system, which consequently performs an initial correction of the working mark.
The measurement of the length of the tool is carried out outside the lathe on an optical or mechanical measuring or test bench, which requires the removal and refitting of the tool which has to pass from the work place to the test bench and vice versa. This transfer considerably reduces precision.
A second method consists of measuring the length of each tool, when the latter is in place on the lathe, using optical or mechanical measuring means occupying a clearly defined position in the fixed mark linked with the machine. This method suffers from the disadvantage of having to be repeated for each tool.
Both these method lack precision, because in both of them the measurements of the tool are only carried out relative to two directions of the working mark. In the best of cases, even if the working edge of the tool is likened to a circle and even if the programming of the trajectory of the tool point belonging to this edge and to the cutting zone is brought to the centre of the circle, any circularity defect of the edge of the tool will lead to an imprecision of the profile to be machined.
In a high precision, digitally controlled lathe, where only one predetermined point of the cutting zone of the tool machines the part, it is necessary to mark the tangents at each point on the profile, said tangents making it possible to orient the tool permanently during machining, so that only the chosen contact point is involved. One of the aforementioned methods makes possible such a precise marking of the tangents.